For example: set up Buy one get the second 20% off, get the third 30% off with bundle:
https://treedify.com/selly/bundles
In bundle offer setting, it needs 3 product groups:
Add/remove product groups to create big bundle
Set bundle discount option "Various discounts":
Set different discounts in different groups:
Group 1 with no discount:
Group 2 with 20% discount:
Group 3 with 30% discount:
* The above setting means customers have to add 3 items to get the offer.
If you want to offer discount when customers buy 2 items, you can also create another bundle of 2 groups: group 1 without discount, group 2 discount 20%.